Internet Protocol Address (or IP Address) is an unique address that computing devices use to identify itself and communicate with other devices in the Internet Protocol network. Any device connected to the IP network must have an unique IP address within its network. An IP address is analogous to a street address or telephone number in that it is used to uniquely identify a network device to deliver mail message, or call ("view") a website.

Dotted Decimals

The traditional IP Addresses (IPv4) uses a 32-bit number to represent an IP address, and it defines both network and host address. Due to IPv4 addresses running out, a new version of the IP protocol (IPv6) has been invented to offer virtually limitless number of unique addresses. An IP address is written in "dotted decimal" notation, which is 4 sets of numbers separated by period each set representing 8-bit number ranging from (0-255). An example of IPv4 address is 216.3.128.12, which is the IP address assigned to topwebhosts.org.

An IPv4 address is divided into two parts: network and host address. The network address determines how many of the 32 bits are used for the network address, and remaining bits for the host address. The host address can further divided into subnetwork and host number.

Class A, B, C and CIDR networks

Traditionally IP network is classified as A, B or C network. The computers identified the class by the first 3 bits (A=000, B=100, C=110), while humans identify the class by first octet(8-bit) number. With scarcity of IP addresses, the class-based system has been replaced by Classless Inter-Domain Routing (CIDR) to more efficiently allocate IP addresses.

Class

Network Address

Number of Hosts

Netmask

CIDR

/4

240,435,456

240.0.0.0

CIDR

/5

134,217,728

248.0.0.0

CIDR

/6

67,108,864

252.0.0.0

CIDR

/7

33,554,432

254.0.0.0

A

/8 (1-126)

16,777,216

255.0.0.0

CIDR

/9

8,388,608

255.128.0.0

CIDR

/10

4,194,304

255.192.0.0

CIDR

/11

2,097,152

255.224.0.0

CIDR

/12

1,048,576

255.240.0.0

CIDR

/13

524,288

255.248.0.0

CIDR

/14

262,144

255.252.0.0

CIDR

/15

131,072

255.254.0.0

B

/16 (128-191)

65,534

255.255.0.0

CIDR

/17

32,768

255.255.128.0

CIDR

/18

16,384

255.255.192.0

CIDR

/19

8,192

255.255.224.0

CIDR

/20

4,096

255.255.240.0

CIDR

/21

2,048

255.255.248.0

CIDR

/22

1,024

255.255.252.0

CIDR

/23

512

255.255.254.0

C

/24 (192-223)

256

255.255.255.0

CIDR

/25

128

255.255.255.128

CIDR

/26

64

255.255.255.192

CIDR

/27

32

255.255.255.224

CIDR

/28

16

255.255.255.240

CIDR

/29

8

255.255.255.248

CIDR

/30

4

255.255.255.252

Note: (1) 127 Network Address reserved for loopback test. (2) Class D (224-247, Multicast) and Class E (248-255, Experimental) are not intended to be used in public operation.

Public and Private IP Addresses

In order to maintain uniqueness within global namespace, the IP addresses are publicly registered with the Network Information Center (NIC) to avoid address conflicts. Devices that need to be publicly identified such as web or mail servers must have a globally unique IP address, and they are assigned a public IP address. Devices that do not require public access may be assigned a private IP address, and make it uniquely identifiable within one organization. For example, a network printer may be assigned a private IP address to prevent the world from printing from it. To allow organizations to freely assign private IP addresses, the NIC has reserved certain address blocks for private use. A private network is a network that uses RFC 1918 IP address space. The following IP blocks are reserved for private IP addresses.

The loopback IP address is the address used to access itself. The IPv4 designated 127.0.0.1 as the loopback address with the 255.0.0.0 subnet mask. A loopback interface is also known as a virtual IP, which does not associate with hardware interface. On Linux systems, the loopback interface is commonly called lo or lo0. The corresponding hostname for this interface is called localhost.

The loopback address is used to test network software without physically installing a Network Interface Card (NIC), and without having to physically connect the machine to a TCP/IP network. A good example of this is to access the web server running on itself by using http://127.0.0.1 or http://localhost.

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